Fix crypto._cert_fingerprint

Ensure first parameter to load_pem_x509_certificate is always bytes

Author: zmousm

Diff for: src/xmlsec/crypto.py

@@ -358,6 +358,8 @@ def _get_cert_by_fp(self, fp):
 
 def _cert_fingerprint(cert_pem):
     if "-----BEGIN CERTIFICATE" in cert_pem:
+        if isinstance(cert_pem, six.text_type):
+            cert_pem = cert_pem.encode()
         cert = load_pem_x509_certificate(cert_pem, backend=default_backend())
     else:
         cert = load_der_x509_certificate(base64.standard_b64decode(cert_pem), backend=default_backend())